In the art of aeronautics, the use of inerting systems is well known for the generation of an inert gas, such as nitrogen or any other inert gas such as carbon dioxide, and for introducing said inert gas into fuel tanks for safety reasons in order to reduce the risk of explosion from said tanks.
A conventional, prior art inerting system typically includes an on board inert gas generation system (OBIGGS) supplied with air, for example with bleed air diverted from at least one engine. The bleed air diverted from at least one engine is currently the most widely used model. In such a system, the bleed air is typically routed from one or more engines at the opening known as the intermediate pressure port and/or the opening known as the high pressure port, depending on the flight situation. It should be noted that the use of bleed air for the air conditioning is advantageous because the bleed air has a relatively high pressure, as well as a relatively high temperature, such that the air can be adjusted to a wide range of desired pressure and temperature settings. The OBIGGS is connected to the airplane fuel tank and separates oxygen from the air.
An OBIGGS typically comprises an air separation module, or several modules arranged in parallel, containing for example zeolite membranes through which an air flow is forced. Due to the different mass transfer rates for nitrogen and oxygen, the system splits the air flow such that an air flow with high nitrogen content and an air flow with high oxygen content are obtained. The air fraction enriched with nitrogen, considered to be the inert gas, is routed into fuel tanks such that the mixture of air and kerosene vapor present at this location is displaced and discharged from the tanks. The air fraction enriched with oxygen may be reintroduced into the passenger cabin after having been treated using appropriate means and/or into the reactors' combustion chamber to improve combustion. The devices required for this process such as compressors, filters, and air or water cooling modules or similar are integrated into the inerting system.
When the ratio between fuel and oxygen in the empty part of the tank is below the ignition limit defined in accordance with the Federal Aviation Administration (FAA) requirements detailed in AC25.981-2A dated Sep. 19, 2008 and entitled “FUEL TANK FLAMMABILITY REDUCTION MEANS” and its appendices, no spontaneous ignition may occur. From the foregoing, inerting a fuel tank notably consists in injecting an inert gas in order to maintain the level of oxygen present within said tank below a certain threshold, for example 12%.
The inert gas generation systems known in the prior art comprise at least two air separation modules, which are arranged in parallel in order to generate and deliver a nitrogen-enriched gas with desired purity, in terms of residual oxygen concentration, and desired flow rate.
The inerting system preferably comprises a flow control valve installed downstream from the air separation modules, in order to modulate the type of flow sent to the tanks to suit the aircraft flight phase.
A low flow modulation rate, for example from 0.45 to 0.90 kg/min, allows for an inert gas of very high quality to be generated, notably comprising about 3% oxygen. This low flow mode is typically used during the stable phases of the aircraft, for example during the ground or cruising phases, which require relatively low inert gas flow rates.
In descent mode, the inerting system tends to use a high flow rate mode, for example from 0.68 to 1.36 kg/min, for which the flow rate of the inert gas sent to the tanks is high but the quality and purity levels are lower, notably of about 13% oxygen.
The main drawback of inert gas generation systems known in the prior art is their size. In fact, the arrangement of the air separation modules results in a generation system that is oversized, for example in terms of the number of modules and filtration components, in relation to the actual flight phase need, which in turn causes excessive consumption of kerosene and an increase in the weight of the aircraft.